Wet surface sensor

Thread Starter


Joined Apr 17, 2008
I intend to design a circuit that senses wetness of a surface and lits an LED and sounds a buzzer. I need help with the probe sensors that are used in this application.

Thank you,
Ray Kusha


Joined Feb 12, 2009
Not quite sure what you mean by "wetness of a surface".

If you want to detect rain drops on a surface then a small area of stripboard with the strips alternatively connected will do. The resistance will drop to a fairly low value when adjacent strips are bridged by a rain drop.

A simple comparator is used to detect between low resistance (rain), high resistance (no rain).

Devices that measure the dampness of walls, for example, have two sharp pins that can penetrate. The resistance between these pins reduces as the dampness increases. Probably best to make the pins from stainless steel.

Here you might want to measure the value of resistance and ultimately drive a display possibly using a micro-controller.

The sensor pins will corrode due to electrolytic action so good quality sensors use AC voltage across the pins or regularly swap the polarity.

Does that help?


Joined Sep 9, 2010
There are also RF devices which measure moisture in walls and floors, concrete for instance, and these devices are not invasive (no probes, essentially no contact). Home inspectors use them.